1. Field of the Invention
The present invention relates to an image reading apparatus having a reading means for reading the image of an original document. The document and the reading means are moved relative to each other by means of a stepping motor, and the image of the original document is read even while the stepping motor is being accelerated or decelerated.
2. Description of The Related Art
Generally, in a stepping motor, the pull-in torque thereof decreases with an increase in the rotational speed thereof. It is impossible to make a motor rotate suddenly at a speed exceeding the minimum self-start frequency. Therefore, when a motor is rotated where the pull-in torque is small or where the rotation speed exceeds the minimum self-start frequency, acceleration control must be performed so that the speed is gradually increased starting from a speed with a large pull-in torque.
Hitherto, an image reading apparatus for reading the image of a document using a stepping motor by causing a document and reading means to move relative to each other and by controlling the acceleration of the stepping motor has been constructed as described below.
The document is set in a document tray in an image reading apparatus in which a document is fed. The document begins to be fed from the set position, and acceleration control is terminated by the time the document reaches the position at which the image of the document is read.
In an image reading apparatus featuring a moving-type optical system in which a document set in a document tray is read by moving the optical system, the optical system begins to move from the home position, and acceleration control is terminated by the time the document reaches the reading position in the document holder.
However, in the related art, when the reading operation is suspended due to a memory overflow or the like during the reading operation, the stopped position and hence, the reading position, is in the middle of the document. The related art has a problem in that if the reading operation is restarted from the position at which the reading is stopped, and if acceleration control for making the reading speed constant from the stopped state is performed, part of the image is skipped because no reading of the original document is performed during acceleration control.
The torque of the stepping motor decreases with an increase in the rotational speed thereof. In the related art, since quick-acceleration control is performed in order to shorten acceleration time, the torque decreases considerably before the document reaches the reading position. As a result, in a document-feeding type image reading apparatus, an auto-document feeder (ADF) mechanism does not function satisfactorily, causing the following problems: (i) double-feeding of the documents, (ii) the motor is out of step when documents are fed which are stacked on one another in the feeding means and which have a large frictional resistance, or (iii) causing defective feeding.